Means for stretch molding of plastic receptacles for packaging such as bottles, wide-mouthed jars or the like include the cold parison method for stretch blowing a preformed bottomed preform into a receptacle by heat softening at a temperature higher than the glass transition point, and the hot parison method for immediately stretch blowing an injection molded soft and bottomed preform into a receptacle, while it maintains a potential heat higher than the glass transition point.
In the cold parison method, it is believed that the preform temperature becomes uniform and the crystalline density of the wall cross-section of the receptacle molded by stretch blowing is also uniformed, because a preform of ambient temperature is heated and softened by an external heat, allowing to obtain a receptacle easy for heat resistant treatment.
In the hot parison method, it is believed that the preform temperature tends to be uneven, because the stretch blowing should be completed while the preform is being softened by the potential heat, and the crystalline density is lower that the cold parison method, making the heat resistant treatment deficient, without any remarkable difference in strength.
The present invention devised in consideration of the aforementioned situation and has an object to provide a novel stretch blow receptacle and molding method wherein the wall cross-section is multi-layered in view of crystalline density, and the crystalline density of the surface layer is made extremely higher that the crystalline density of the interior, thereby allowing to intend to increase the strength of the receptacle and to make the heat resistant treatment easier.